The present invention relates to a method and a device for detecting oscillatory failures in at least one automatic position control chain of an aircraft control surface, as well as a system of electrical flight controls comprising such a detection device.
The present invention applies to an automatic control chain:                which is intended to automatically control the position of all kinds of aircraft control surface, such as ailerons, spoilers or an elevator, for example;        which forms part of a system of electrical flight controls of the aircraft; and        which comprises:        
the control surface which is mobile, and the position of which relative to the aircraft is set by at least one actuator;
the actuator which sets the position of the control surface, according to at least one actuation order received;
at least one sensor which measures the actual position of the control surface; and
a computer which generates a control surface actuation order, transmitted to the actuator, from the measured actual position and a control order computed by the automatic pilot or from the action of the pilot on a control column and on the inertial state of the aircraft.
It is known that such an automatic control chain comprises electronic components which are likely, in failing mode, to generate a spurious signal which can cause the automatically controlled control surface to oscillate. A phenomenon of this type is called “oscillatory failure.” Another possible cause of the oscillation is the malfunction or the breakage of a mechanical part of the actuator.
It is also known that, when such an oscillatory failure exhibits a frequency which is located within the bandwidth of the actuator, it has the effect:                of generating significant loads on the structure of the aircraft, which makes it necessary to reinforce this structure, if a dedicated device does not make it possible to detect this failure;        of generating loads in case of excitation of one of the fundamental modes of vibration of the aircraft (phenomenon of resonance, aeroelastic coupling);        of accelerating the fatigue of the structure of the aircraft;        of accelerating the fatigue of the actuator or actuators used; and        of reducing the comfort of the passengers of the aircraft.        
In practice, the aircraft is designed to support a certain load envelope which is defined by an amplitude/frequency curve. Thus, for example, the appearance of wind gusts will impact the flexible structure of the aircraft and generate loads on this structure but, with the aircraft being designed to withstand these loads, there are no particular actions to be put in place. However, in the case of an oscillatory failure, it may be that the associated loads lie outside the design envelope. Specific monitoring procedures are put in place to rapidly detect these stray oscillations and thus guarantee that the vibrations of the aircraft remain within the predetermined amplitude/frequency envelope. In the absence of these monitoring means, the complete coverage of such oscillatory failures would require structural reinforcements of the aircraft, which would increase the cost of the aircraft.
However, the usual solutions for performing such monitoring procedures exhibit a high dependency in relation:                to the hardware used;        to the type of piloting law of the aircraft (function of the flexibility or non-flexibility thereof);        to the acquisition and generation system of the computer; and        to the failure modes of the computer.        
Consequently, a particular family of aircraft each time has a corresponding particular usual solution, which does not offer any guarantee of being applicable to another family of aircraft, whether existing or future.
Furthermore, the usual monitoring solutions generally have a restricted coverage, by more often than not performing only a detection of the oscillations generated by a particular component of the automatic control chain.
The patents FR-2 893 911 and FR-2 936 067 make it possible to at least partially remedy these drawbacks. The patent FR-2 893 911 notably provides for comparing the real operation of the automatic control chain monitored (which is illustrated by the measured actual position), to a failure-free expected ideal operation (which is illustrated by a theoretical position obtained from a model), which makes it possible to reveal any oscillatory failure when it occurs.
The model-based monitoring procedures have demonstrated their benefit and their applicability to the detection of oscillatory failures. By definition, they rely on the knowledge of a model, specific to each type of actuator, and their performance levels depend on the quality of these models (representativeness, uncertainties, model noises, etc.). However, a method developed for a conventional hydraulic actuator, with a model specific to this type of actuator, cannot be adapted to another type of actuator (for example of EHA, Electro-Hydrostatic Actuator, type), the physical principle and the operation of which are governed by different principles, and which is therefore described by a different model. Adaptive algorithms (for example of EKF, Extended Kalman Filter, type) can compensate these model differences, but only to a certain extent. It is therefore necessary to develop a model for each type of actuator and, in practice, some types of models lend themselves better to model-based failure detection methods than others. Thus, a physical model is well suited to observer-type techniques whereas a model of transfer function, or transfer function network type, is more difficult to use for this type of method.